PoliCTF 2015 - It's Hungry

It’s Hungry

Forensics - 100 Points

Old McDonald had a farm. Old McDonald liked chiptune. He also needed to remind its daughter to take care about a zombie animal. But he wanted to do it discreetly, so he wrote this song. Can you find the message? (all lowercase, no spaces) N.B. flag is not in format flag{.+}

Hidden/Special Challenge!

Writeup

This one was a hidden challenge available clicking on the timestamp of the chiptune player.

phosphore:~$ wget http://polictf.it/tunes/oldmcdonald.flac

We started by inspecting the sources of the js player but we found nothing. So we focused on the audio file oldmcdonald.flac.

Again, using metaflac, a command-line FLAC metadata editor: no results.
Being aware of several ways to hide messages in audio files we tried to extract text by LSB-based audio steganography. Failing.
That’s when we opened up the spectogram of the song with sonic-visualizer…

then, around 1:08 at ≃ 18’600Hz, we run into this:

Clearly morse! After a few tries this happened.

Yes, we were frustrated, so we vented against ocean, a #polictf op :(

“just listen” said the hint. We figured that we were supposed to listen to the chiptune. At the end we tried writing the notes down and we came up with f e d a d e a d b d e, which made no sense.
Alas, turns out we got the notes wrong and they instead were f e e d d a d e a d b e e f, our flag.